Integral transformer electromagnetic transducer



June 24, 1958 D. F. BROWER EIAL 2,840,643

INTEGRAL TRANSFORMER ELECTRQMAGNETIQ TRANSDUCER Filed April 4. 1955 2 Sheets-Sheet 1 [Mara/r. 01/40 A axon 1!,

Y a 1 122254. v W W June 24, 1958 ow ETAL 2,840,643

INTEGRAL TRANSFORMER ELECTROMAGNETIC TRANSDUCER Filed April 4, 1955 2 Sheets-Sheet 2 minimum of strayleakage flux.

Unite INTEGRAL TRANsFoRMnn ELECTROMAGNETIC TRANSDUCER Application April 4, 19ss,seria1No.'499,142-

Claims. (Cl. 179-1001 The present invention relates to electromagnetic transducers, and more particularly to an improved electromagnetic head for recording, reproducing or erasing information stored as states of magnetization on lengthy magnetizable record bodies. 7 a p i a In copending U. S. patent application, Serial No. 454,551, filed September 7, 1954, and entitled, Electromagnetic Transducer, by David F. Brower, there is dis closed 'a novel form of single-turn electromagnetic transducer or head characterized by an operating efficiency and ease of fabrication heretofore unknown in the art. The single turn feature of the transducer substantially contributes to the efficiency of coupling between the transducer and record body. However, because the single-turn has a low impedance, it is generally necessary to provide a transformer for coupling the transducer to anexternal circuit. 7

The use of an external coupling transformer, such as that shown in the cited Brower application, considerably increases the overall space required for the transducer combination. In addition, the use of a transformer external to the transducer requires that two separate magnetic structures be utilized with a consequentincrease in the losses, and a consequent reduction in efliciency.

It is, therefore, an object of the present invention to I provide an improved electromagnetic transducer or head having an integral coupling transformer provided therein.

Another object of the present invention is to provide an electromagnetic transducer and coupling transformer in which a common magnetic core is utilized for both transducer and transformer.

A further object of the present invention is to provide an improved electromagnetic transducer having a con- Ipling transformer provided therein characterized by. an :J'inher'ent simplicity and'ease of fabrication heretofore irnpossibleof-achievement.

Still'another object of the, present invention is toprovide an electromagnetic transducer with a coupling transforinerbiiilt .the'reinhaving a novel coreconfiguration which 'p'er'mitsthe. same magnetic core to be utilized for "both transducer and transformer, and yet provides an optimallyjeificient electrical 'and magnetic structure-for each'function. a

Yet anotherfobjectofthe present invention is to pro- Vide an'electr'oniagnetic transducer and transformer having acor'ri'mon core structure, a. portion of which serves as the core for the tr ansduce'rand another portion a-sthe core for the transformer, which provides for optimum coupling "of the transducer and the tIanS'former with a "An electromagnetic transducer, according to the'p'rese ent'invention, comprises a transformer'core, a first endless winding disposedon the core to form an open 'tilrn about the core with a segment ,of the windingdisposed to have an edge fius'h with the surface of an outer edge of fhe'core, and as'ec'ond winding wound over aportion of the first winding.

States Patent O 2,840,643 Patented June 24, 1958 .2 The novel features which are believed to be character- "istic of the invention, both as to its organization and method'of operation, together with further objects and .advantages thereof, will be better understood from the -descriptionconsidered in connection with the accompanying drawings in which an embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and'description only, and are not intended as a definition-of the limits of the invention.

.Fig. 1 is a perspective view, partly in section, of the electromagnetic transducer and transformer of thepresent invention; Fig. 2 is a viewin perspective of a conductive shim which may be utilized in the transducer of, the present invention; 2

- Figs. 3a and 3b are elevational views illustrating various shapes of the transducer pole faces to adapt them'for different record bodies;

Fig. 4 is a perspective view of the embodiment of 1, illustrative of flux and current paths Within the transducer;-and a a 1 Fig. 5 is a view in perspective of a multiple transducer array, according to the present invention, which includes a plurality of individual transducers for recording afplurality of parallel magnetic tracks. Referring now to the drawings, wherein like reference characters designate like or correspondingpartsthroughout the several figures, there is shown in Figal oneem- 1 0 at its pole-face end, the portion of ribbon '12 lying between the core pieces having an-edge 13 disposed in the plane defined by the adjacentedges 14 and 15 of .pole faces 110 and 111, respectively. A secondfiat conductive ribbon 17 substantially encircles core piece 10 in a region intermediate the region covered by conductor 12 and the line of contact'16 between core pieces wand 11, whilethe ends of ribbon 17 and conductor 12 are interconnected by means of a pair of flat conductive bars 18 and 20,-respectively. Conductive bars 18 and 20 extend along and parallel to core piece 10, and are'superposed along at least a portion of their width, the superposed ,..;portions, as'well as the ends of conductor 12 and ribbon 17, being insulated from each other by insulating 'r'n'a- I -,te,rial', represented asa strip 21.

Ribbon 17 serves as the primary winding of a conpling tr ansformer-22, the secondary'windingfof the'transfofr'n'er'co'mprising a plurality of turns of relatively fine wirewound thereover and depicted ascoil 23. The ends orterfninals of coil 23, numbered 24 andr'ZS respectively,"arejutiliiedforinterconnecting the transducer withfan fext'eifial f circuit, not shown, for alternately exciting the [transducer or deriving signals therefrom. Core piece "11,

; liich"h as one side in intimate contact with conductor "12,'h'asa'recess19cut therein opposite coil 23 to permit Ttheportion of the core extending beyond the coil to'make contact alongjlin'e 16 with a similarpor'tion of core piece "turn about the core piece.

transformer thus described,*core pieces and 11 preferably are of a material having highrnagnetic permeability, low magnetic retentivity, and high specificresistivityy Somematgnetoceramicwmaterials display these properties, and accordingly the core pieces may be of magneto-ceramic material, such as the ferrite materials.

Core pieces 10 and 11 may initially be fabricated to identical'dimensions to as close tolerance as required,

using techniques well known in the art, following which recess19 having. appropriate dimensions may be cut in core piece 11. The use of core pieces having a high specific resistance obviates the necessity of including insutransformer.

1 'jFlat ribbon conductor 12 maybe of copper or silver, orsimilar highly conductive material, of 1 to 2 mils thickness, the thickness of the conductor serving to establishfthewidth of therecording gap in the magnetic structure. Alternatively, the ribbon may be of any other, thickn'ss, the selected dimensions being based onconsiderations well known in the magnetic recording art.

Conductive ribbon 17 and conductive bars 18 and 20 may be composed of similar material of the samethickness, if desired. an: efliciency of the transformer portionof the transducer may be considerablyenhanced and stray fluxfpaths" substantially reduced by dimensioning'the widthof conductive ribbon 17 and coil 23 so thatthe coil completely covers the ribbon.

While conductor 12, ribbon 17 and bars 18 and 20 have beendes'cribed as separate parts and may be so fabricated,.the interconnection of the parts and their final assembly in the form shown in Fig. 1 may be considerably simplified by initially fabricating the parts from a single piece of highly conductive material, shapedsubstantially as shownin Fig. 2. As shown in Fig. 2, conductiveshim or band 26 is asingle piece of substantially rectangular highly conductive materialhaving an aperture 27 cut out therein. Shim 26 has a length corresponding to the desired final distance between edge 13 and the edge of ribbon 17 adjacent contact line 16, and a width at least slightly greater than the circumference of core piece 10.. Aperture 27, which may have a width substantially corresponding to the width of the adjacent sides of core pieces 10 and 11 and a length selected to produce the desired width for each of conductors 12 andfribbon 17, 'ispositioned with respect to the side edges of shim 26'soas to fall between the adjacent sides of .core pi eces 10 and 11 when the parts are assembled in their final form. It should be noted that aperture 27 need notbe placed in the middle of shim 26, when it is that conductive bars 18 and 20 overlap along .a sideof core piece. 10 other than the side opposite the side adjacent core piece 11.

are preferably cut to provide that edge 13 have a length corresponding to the width (of the recording gap the core structure, and are particularly u'sefulto reduce the effect of current flowing in these portions of the'shim on the record body. Shaping the shim in this manner also reduces the danger of shorting the edges of the shim if the poleface end of the transducer is to be shaped 'or groundfollowingassembly. y,

lation between the core piecesand the normally current conductive elements of the transducer such as conductor t 12 and ribbon 17. Thus, each of these elements is placed in intimate contact with the core structure, thereby in- "creasing the magnetic efficiency of the transducer and Considering now the assembly of core pieces 10 and 11 and shim 26, core piece 10 may first be placed on shim 26 with its pole-face edge 14 adjacent shim edge 13 and the core piece lying on the shim parallel to its length. The portions of shim 26 not covered by core piece 10 may then be folded about the core piece, with a strip 21 of insulating material interposed between the oyerlapping portions of the shim in order to electrically insulate the overlapping portions of the shim from each other. Coil 23 may then be wound about core piece 10 and shim 26, and core piece 11 may then be placed in proper abutting relationship with the assembly thus formed. The resulting transducer may be encapsulated in plastic, or placed in any other type housing which may retain the parts in appropriate relationship.

For recording intelligence, a translatable record body may be caused to pass successively over pole face 110, edge 13, and pole face 111, electrical current representing the data to be recorded meanwhile being applied to the transducer by means of leads 24 and 25 attached to coil 23. The transducer of the present invention may be used either for contact recording, that'is, with the transducer in contact with the record body, or for noncontact recording, that is, with the transducer merely in close proximity to but spaced from the record body. Preferably, the record body may be moved at right angles to the portion of shim 26 which is between pole faces 110 and 111. The transducer may be employed in a similar manner for reading or picking up information which has already been recorded, or for erasing, in accordance with systems well known in the art.

After completing the assembly of the transducer, the pole-face ends may be shaped to any desired contour, thereby improving the operational characteristics of the transducer. For example, as shown in Fig. 3a, where the transducer is to be employed for recording on a cylindrical drum surface 31, the pole-face end 32 of the transducer may be ground to a concave shape of approximately the same radius as that of the drum. Such shaping has been found to be of value when the transducer is utilized in non-contact recording, and of especial value when the transducer is utilized in contact with the drum surface. Where the transducer is to be employed in conjunction with a flexible magnetizable tape 33, the pole-face end 32 of the transducer may be ground generally in V shape, and the tape drawn across the poleface end of the drum in tension, as shown in Fig. 3b. Under such conditions, folds and creases in the tape tend to be ironed out as the tape passes across the transducer, and the reliability of operation is substantially improved;

' Referring now to Fig. 4, there is shown a diagram illustransducer of Fig. 1 when a record body 33 is moved past the pole-face end of the transducer. As shown in Fig. 4, motion of the previously recorded magnetic areas on record body 33, in the direction of the arrow,,past the pole-face end 32 of the transducer will cause stray flux to flow into the transducer. The high permeability of core pieces 10 and 11 will cause this stray flux to tend to encircle conductor 12 as shown by the heavy-dashed line 34. The direction of flux flow will arbitrarily be assumed to be as represented by the arrows on the line. As the record body moves, flux 34 will tend to cut conductor 12 inducing a current therein, the path of current flow being depicted by the lightly dotted line 35, the direction or flow again being. indicated by the arrows. The current path 35 will follow a path of flow through conductor 12, down bar 18, through ribbon 17 and return to conductor 12 through bar 20. It will be seen that the current flowing through the ribbons and the bars passes around core piece 10 in ribbon 17 in a direction opposite to the direction of current flow through conductor 12. The current flowing through ribbon 17 will tend to produce flux in the core pieces following the path depicted be confined to the core pieces due to their relatively high permeability as compared to the conductive parts of the transducer and free space. Flux 36 will in turn tend to induce currents in coil 23 as it intersects the coil. These currents may then be utilized in an external circuit by means of connections to coil ends 24 and 25.

It willthus be seen that the path of current in conductive shim or band 26, comprising conductor-12,' bars 18 and 2t) and ribbon 17, tends to fiow about core piece in a path substantially corresponding to the outline of a saddle disposed on core piece 10, where one hump'of the saddle is disposed in the recording gap of the core structure and coil 23 is wound about another hump .of the saddle-shaped conductive path.

It will be also be seen that flux 34 and flux 36 tend to traverse the regions of core pieces 10 and 11 intermediate conductor 12 and ribbon 17 in opposite directions, and, accordingly, this flux will tend to cancel in this region. In practice, a flux corresponding to a single turn times the current flowing in shim 26 Will pass through the aperture. Since this flux will always have a small value, the reluctance of the magnetic path in this area need not be low and accordingly core pieces 10 and 11 need not be in contact in this intermediate region. In fact, where the transducer and transformer of the present invention is to be utilized at relatively high frequencies, aperture 27 in shim 26 may be omitted from the shim with no apparent reduction in the operating efiiciency of the transducer.

By way of further explanation of the mode of operation of the present invention, it should be noted that any record body flux which tends to traverse the entire length of the core pieces passes through coil 23 in the same direction as the flux produced by the current flowing in ribbon 17, and accordingly, augments the current inducing flux produced thereby. While the presence of an aperture in conductive shim 26 contributes substantially to the performance of the transducer at low frequencies, at higher frequencies where the aperture is omitted, the path of stray flux from the record body may be considered to be from the record body into pole face 110, thence along the entire length of core piece 10, over to core piece 11 and then along the entire length of core piece.

11, returning to the record body through pole face 111.

The advantages of the transducer of the present invention in manufacture and assembly over the prior art transducers will at once be readily apparent. In particular, the transducer may be readily scaled to any desired dimensions. Thus the transducer may be made as small r as desired without unduly complicating either its fabrication or assembly. It will also be noted that the tolerance of the dimensions of the head is determined almost completely by the tolerance maintained in shaping the core pieces. Since these pieces are merely oblong rectangular pieces, their fabrication to close tolerances will present no unusual problems.

Similarly, it will be noted that a further advantage of the construction thus described is that the planeness of the active portion of conductor 12 in the gap between core pieces 10 and 11 is determined by the planeness of the portions of core pieces 10 and 11 abutting the conductor. Since the core pieces are essentially rigid elements, and may be readily machined to the planeness required, planeness of a high order may be imparted to conductor 12. The use of a malleable material, such as silver, for conductor 12, contributes to the achievement of this advantage, the ribbon readily conforming to the shape required when compressed between the two core pieces.

A further advantage of thet present transducer is that the location of the gap with respect to the external mounting surfaces of the transducer is again purely a function of the shape of the core pieces, and since the core pieces have regular geometric shapes, their fabrication to a high order of accuracy, and consequently, the accurate posimay additionally serve as shielding means for 6 tioning of the completed transducer with respect to the tape or drum, may be readily achieved.

' Still another advantage of the embodiment of the'transducer shown in Fig. l-is that the width of the transducer is only slightly greater than the width of the track of the tape or drum. Accordingly, additional transducers may readily be placed closely 'adjacent to each other in order-to provide for the. recording of closely spaced parallel tracks on the same tape or drum. As will be more fully discussed hereinafter, the' transducer of the present invention is particularly useful for such applications.

In operation the transducer and transformer'of-the present invention olfers a number of advantages over conventional coil type transducers, as well as over the singleturn type transducers heretofore proposed. In particular shim 26, in addition to the current conductive functions heretofore set forth, performs a self-shielding" function and substantially reduces stray flux which might otherwise arise where the transducer is of a relatively open-type construction. 'Where coil 23 has a widthcorresponding to the width of ribbon ,17, the coupling between these two elements becomes unity for practical purposes, and accordingly, the transformer portion of the present invention produces practically zero stray flux.

Besides reducing the effective inductance of the transducer-transformer confiugration, the self-shielding properties also tend to contribute to the electrical isolation of the transducer from its surroundings due to the elimination of stray 'fluxfields.

The transducer of the present invention is particularly useful where a plurality of transducers is disposed for simultaneously recording signals on a plurality of parallel record paths of a single tape or drum. A particular advantage of the present transducer in such applications is the fact that the means for mounting the transducers both the transducer and transformer.

An example of such a configuration which provides for two transducersfor the parallel recording of two recordtracks, is shown in Fig. '5. The transducer array of Fig. 5 includes first and second transducers 50 and 50, a common mounting and shielding block 51 and two retaining strips 52 and 52' for retaining the transducers within mounting and shielding block 51.

More particularly, mounting and shielding block 51 is a block of rigid conductive material such as copper or brass, having two slots, 53 and 53' in one edge thereof. Slots 53 and 53' are cut as by milling to a depth appropriate for positioning each transducer. The slots are of suflicient depth and Width to receive transducers 5t) and 50 respectively and accordingly include an additional cutaway for receiving the exciting coils of the transducers. After positioning the transducers within their respective slots, retaining strips 52 and 52 may be placed over each transducer and fastened, as by soldering or similar techniques in order to retain the transducers within their respective slots. The two retaining strips may be of material similar to block 51. A strip of paper or similar insulating sheet material may preferably be placed between the normally current conductive parts of the transducers, and block 51, and the retaining strips in order to prevent short-circuiting of the transducers.

Following assembly of the transducers, mounting block and retaining strips, the pole-face end of the as sembly may be shaped by grinding to any desired contour for presentment to a tape or drum, as previously discussed. As shown in Fig. 5, the pole-face end may be generally of V-shape, as for example where the array is to be used for recording parallel tracks on tape.

The embodiments of the present invention thus described represent preferred embodiments. It should be understood, however, that numerous modifications may be made. For example, in Fig. 1, where conductive inga portion of said first winding.

material, the, desired configuration may readily be achieved by depositing a conductivecoating of appropriate shapendirectly, on core piece 10, utilizing tech- ,lniques well knownin the art. i k i What-isclaimedis:

.l An electromagnetic transducer comprising a trans- ,former core, a first endless winding on said core, said first winding being disposed aboutsaid core in such manner that the 'windingldoes not form a closed loop about -sa idlcore, and a second, winding wound over a portion of t said first winding,said firstiwinding including a segment disposed on said'core in such manner asto have an,

edge flushwiththe surface of an outer edge of said core.

2. An electromagnetictransducer comprising a transformer core, a gap .in' the peripheral face ;of said core, a first:.winding on said core, said windingrbeing endless, said endless winding being arranged about said-core in such manner as to form anopen turn about said core andhaving a portion of one edge disposed'in said gap, anda second winding wound over said core 3. In combination in an electromagnetic tra nsducer head, a magnetic core havingrrecordconfronting surfacesnwith an air-gap therein for permitting travel of a record member therebefore, means defining a window to permit mounting an energizing coil on said core, a

non-magnetic electrically conducting shield member having one portion filling at least a portion of said air-gap and another portion extending into a substantial portion ofsaid window, said shield member being shaped to prevent the forrnation of a short-circuited turn about said core, and an energizing coil wound about said core and said another portion.

4."An;electromagnetic transducer comprising: a first core piece, a flat band of conductive material encircling said first core piece, the ends of said, band overlapping along one side of said first core piece, a sheet of insulatin material forinsulating the ends of said .band from each other, said band having one edge disposed on said core in suchmanner as to be flush with the surface of an outer edge of said core, and an energizing coil wound 'over a portion of said band.

The electromagnetic transducer defined in claim 4,

including a second core piece disposed adjacent to said edgeof said band and said first core piece.

.and-cover- 6. The electromagnetic transducer defined in claim 5,

wherein said band has an aperture in the portion of said band lying intermediate said first and second core pieces,

said energizing coil being dimensioned and disposed so as not to cover said aperture. v

7. An electromagnetic transducer comprising: a pair of core pieces having adjacent pole faces; a fiat ribbon conductor encircling one of said core pieces at itspoleface end, said ribbon conductor passing between the core pieces and being arranged to have an edge lying in a wire wound over said second conductive ribbon, and

means for electrically interconnecting? said ribbonconductor and said conductive ribbon.

8. The electromagnetic transducer defined in claim 7,

wherein said last named means includes a pair of fiat superposed conductors interconnecting said ribbon conductor and said conductive ribbon, said superposed conductors being electrically insulated from each other and being disposed along one side of said one of said core pieces.

, 9. In combination in an electromagnetic transducer head, a magnetic core having record confronting surfaces with, an air-gap therein for permitting travel of a record member therebefore, means defining a window to permit mounting an energizing coil on said core, said window communicating with said air-gap, a non-magnetic electrically conducting shield member having one portion filling at least a portion of said air-gap and another portion extending into a substantial portion of said window, and an energizing coil wound about said core and said another portion. 7

10. In combination in an electromagnetic transducer head, a magnetic core having concave record contacting surfaces with an air-gap therein for permitting travel of a record member thereacross, means defining a window to permit mounting an energizing coil on said core, said window communicating with said air-gap, a non-magnetic electrically conducting shield member having one portion filling at least a portion of said air-gap and an- 

